Emmaline Saunders1, Michael Stearns1, John M.
Bartley2, John R. Bowman2
1. Utah Valley
University, Orem, Utah; 2. University of Utah, Salt Lake City,
Utah
Pluton - Simplest terms, an ancient magma chamber
Stock - The feeder that transfers the magma to be erupted
Textures - Rock distinctions
Equigranular - Equal grains distributed equally
Porphyritic - Not equal grain distribution (opposite of equigranular)
Mafic - A chemical distinction in the rocks (the dark colored minerals)
Felsic - The other end of the chemical distinction in the rocks (the light colored minerals)
Emplaced - A term for magma accumulation in the pluton
Plagioclase, Quartz and Orthoclase - Minerals found in the rocks that can be clasified as mafic or felsic
BSE (Back Scatter Electron) - A type of imaging on an SEM that deals with the average atomic numbers of chemicals
EBSD (Electron Back Scatter Diffraction) - A type of analysis on an SEM that looks at small tiny deformations in the atomic structure of a crystal
The Alta stock, UT, is part of the Wasatch Intrusive Belt and most well-known for its surrounding metamorphic aureole. Baker (1965) defined two textural units within the Alta stock: an equigranular, more mafic border phase and a porphyritic, more felsic central phase. The contacts between the two phases varies from sharp to gradational. Zircon and titanite U-Pb dates range from ~35–31 Ma and young inward suggesting the margins of the stock were emplaced before the central phase.
Plagioclase (Plg), quartz and orthoclase (Qtz+Or) mineral modes from 40 Alta stock samples show a negatively sloped linear trend (R2 = 0.86) between equigranular samples with high Plg/low Qtz+Or samples and and an aplitic sample with low Plg/high Qtz+Or.
Porphyritic samples lie between the two end members along the linear trend. Plagioclase within the equigranular samples form an interlocking framework with abundant Plg-to-Plg grain boundaries.
Crystal plastic deformation (e.g., pressure solution, undulose extinction, mechanical twinning of Plg, and kink bands in biotite) of early-crystallizing phases is more common within the equigranular samples.
Plg in the porphyritic samples are more often solitary phenocrysts surrounded by a matrix of fine-grained Qtz+Or, and have fewer Plg-to-Plg contacts.